Residents of the villages near Volcán de Colima, which marks the border
between the Mexican states of Colima and Jalisco, know what it's like to live
in the dangerous shadow of the most active volcano in Mexico, which has erupted
more than 40 times since 1576. But this week's eruption exceeded the magnitude
of all explosive events in the last 20 years. And if history repeats itself,
some geologists fear the recent activity could be only the beginning of a larger
event to come - one that could parallel or exceed the last major explosive eruption
in 1913, which turned the summit into a 500-meter-deep crater and produced ash
falls as far as 700 kilometers to the northeast.

Volcàn
de Colima (left) emits steam even on a relatively calm day. Nevado de Colima
(right) is not active like its neighbor, but remains the taller of the two at
4,240 meters above sea level. Photo copyright of James Allan, 1981 (Smithsonian
Institution).

At about 11 p.m. local time Monday, June 6, Colima volcano erupted, producing
an ash plume that rose up to 5 kilometers (16,400 feet) above the 4-kilometer-tall
volcano, and sending pyroclastic flows of ash and rock down the flanks. The
plume spread and blanketed nearby villages, Tonila and San Marcos, in a layer
of ash, according to an AP report. Evacuation was not mandatory, but officials
restricted access within an 11.5-kilometer radius from the summit and warned
residents 30 kilometers away in the capital city of Colima to wear masks.

"The eruption Monday sent a lot of ash into the city, causing respiratory
problems," says Jim Luhr, director of the Smithsonian Institution's Global
Volcanism Program. "But that's the least of their worries." What researchers
and the surrounding communities fear most is a collapse of the volcanic cone.

The volcano, sometimes also called Volcán Fuego (or "Fire Volcano"),
rises from a 5-kilometer-wide horseshoe-shaped caldera at the western end of
the Mexican Volcanic Belt. Repeated eruption of lava flows and pyroclastic material
over thousands of years built the volcano's conical shape, but collapse events
have created an "apron" of avalanche deposits around the southern
base. Luhr says that the city of Colima and many towns to its north are all
built on earlier deposits, and might be threatened by a future collapse. The
caldera in which the modern cone sits is the scar left from one or more of the
most recent collapses.

According to Luhr, many collapses have occurred in the history of Colima, the
last one 3,500 years ago. "The active cone is probably now as high as it
has ever been," Luhr says. Also, researchers worry that heated water interacting
with the once solid rock of the cone can weaken it, as indicated by pink and
yellow coloration in the rocks. "This commonly feared pastel coloring,"
Luhr says, "indicates weakness in the deposit and the potential for a collapse."

But a collapse of the mountain is the worst and least likely of three possible
future scenarios evaluated by the University of Colima's volcano observatory.
A second scenario is a continuation of the activity that has occurred since
1998, with additional eruptions similar to Monday's that will drop ash on nearby
communities and generate pyroclastic flows that travel out to about a 5 kilometer
distance. A third scenario involves an enormous eruption on the scale of the
1913 event. Even in this case, however, the city of Colima about 45 kilometers
south of the volcano would not likely be in much danger, Luhr says, because
typical wind patterns move north. "But anything can happen and winds can
change," Luhr says. The cloud from Monday's eruption moved southwest.

Geologists look to the composition of ejecta during eruptive episodes as one
way to evaluate the potential threat. The current episode of activity began
in 1998 and has included the growth of lava domes, flows of lava and ash, and
multiple eruptions. Researchers found that new magma had been produced during
these events, Luhr says. But for an explosion on the scale of the 1913 event
to occur, the magma must retain its water until it reaches the surface. Since
1913, water vapor has always beaten the magma to the surface. "You could
see gases streaming out before the magma," he says. The eruptions to date
have been powered by the expansion of heated groundwater.

Colima's behavior appears to by cyclical, Luhr says, and current events are
similar to those that preceded the 1913 explosion. The question remains as to
why the magma sometimes retains gas up to the point of eruption and, other times,
the gas bleeds out prior to eruption, he says. "We still don't know if
Colima's current magma is gas-rich or not, but evaluation of recent ejecta is
underway to address that question." Ejecta with a glassy texture and is
full of holes, for example, would imply that gas-charged magma has reached the
surface and that Colima has "entered a new phase of activity," Luhr
says. "The people surrounding Colima have quite a dangerous situation on
their hands."

In the meantime, the University of Colima's volcano observatory issued reports
that local villages should remain on alert and under voluntary evacuation. Some
chose to leave, but many have grown wary of multiple evacuations during the
period of high activity beginning in 1998. Monday's eruption was just another
event in "a long and tortuous experience for these people," Luhr says.